1 Field of the Invention
The present invention relates to a process for preparing acrolein or acrylic acid or a mixture thereof by heterogeneously catalyzed partial gas phase oxidation of propene, in which a starting reaction gas mixture which comprises the propylene and molecular oxygen reactants and the inert molecular nitrogen and propane diluent gases and comprises the molecular oxygen and the propylene in a molar O2:C3H6 ratio of ≧1 is conducted at elevated temperature through a fixed catalyst bed whose active composition is at least one multimetal oxide comprising the elements, Mo, Fe and Bi.
2 Description of the Background
Acrolein is a reactive monomer which is especially significant as an intermediate, for example in the preparation of acrylic acid by two-stage heterogeneously catalyzed partial gas phase oxidation of propene. Acrylic acid is suitable as such or in the form of its alkyl esters, for example for preparing polymers which may find use as adhesives or water-absorbent materials among other uses.
The preparation of acrolein by the process, described in the preamble of this document, for the heterogeneously catalyzed partial gas phase oxidation is known (cf., for example, EP-A 11 06 598, WO 97/36849, EP-A 293 224, WO 01/96271, DE-A 198 37 517, EP-A 274 681, DE-A 198 37 519, DE-A 198 37 520, EP-A 117 146, WO 03/11804, U.S. Pat. No. 3,161,670, WO 01/96270, DE-A 195 08 558, DE-A 33 13 573, DE-A 102 45 585, WO 03/076370, DE-A 103 16 039, WO 04/031106, and the German application DE-A 10 2004 032 129 and the prior art cited in these documents). Typically, it forms the first stage of a two-stage heterogeneously catalyzed gas phase partial oxidation of propene to acrylic acid. In the first reaction stage, the propene is substantially partially oxidized to acrolein and, in the second reaction stage, the acrolein formed in the first reaction stage is substantially partially oxidized to acrylic acid. It is significant in this context that the industrial embodiment is normally configured in such a way that the acrolein formed in the first reaction stage is not removed, but rather conducted into the second reaction stage as a constituent of the product gas mixture leaving the first reaction stage, if appropriate supplemented by molecular oxygen and inert gas, and if appropriate cooled by direct and/or indirect cooling.
The target product or the heterogeneously catalyzed gas phase partial oxidation of propene to acrolein is acrolein.
A problem in all heterogeneously catalyzed gas phase partial oxidations in a fixed catalyst bed is that the reaction gas mixture, as it flows through the fixed catalyst bed, passes through a maximum value, known as the hotspot value. This hotspot value is composed of the external heating of the fixed catalyst bed and of the heat of reaction.
For reasons of convenience, the temperature of the fixed catalyst bed and the effective temperature of the fixed catalyst bed are therefore distinguished from one another. The temperature of the fixed catalyst bed refers to the temperature of the fixed catalyst bed when the partial oxidation process is being performed, but in the theoretical absence of a chemical reaction (i.e. without the influence of the heat of reaction). In contrast, the effective temperature of the fixed catalyst bed refers to the actual temperature of the fixed catalyst bed including the heat of reaction of the partial oxidation. When the temperature of the fixed catalyst bed is not constant along the fixed catalyst bed (for example in the case of a plurality of temperature zones), the term temperature of the fixed catalyst bed means the (numerical) average of the temperature along the fixed catalyst bed. The temperature of the fixed catalyst bed over its length may of course also be configured in such a way that the temperature is constant over a certain length, then changes abruptly and maintains this new value over a further length, etc. In that case, reference is made to a fixed catalyst bed (fixed bed catalyst charge) having more than one temperature zone (or else reaction zone), or disposed in more than one temperature zone (or else reaction zone). Catalyst-charged reactors which implement such temperature zones (or else reaction zones) are correspondingly referred to as one-zone or multizone reactors (cf., for example, WO 04/085369). In common with the temperature of the reaction gas mixtures, the effective temperature of the fixed catalyst bed likewise passes through the hotspot value in flow direction of the reaction gas mixture.
One aim of the heterogeneously catalyzed partial gas phase oxidations of propylene to acrolein is thus to configure the hotspot temperature and with it its sensitivity toward an increase in the temperature of the fixed catalyst bed in a very favorable manner. Thus, excessively high hotspot temperatures generally reduce the lifetime of the fixed catalyst bed and the selectivity of target product formation (here: acrolein). Lower hotspot temperatures are normally accompanied by a reduction in the sensitivity thereof toward an increase in the temperature of the fixed catalyst bed. For example, when the partial oxidation is carried out in tube bundle reactors, this is found to be favorable when the individual catalyst tubes, as a consequence of temperature gradients existing over the reactor cross section, cause different temperatures of the catalyst bed disposed in the particular catalyst tube. A presence of propane in the reaction gas mixture results in lower hotspot temperatures as a consequence of the comparatively increased specific heat of the propane (cf., for example, EP-A 293 224).
In addition, a presence of propane, owing to its combustibility, promotes the explosion behavior of the reaction gas mixture (for example DE-A 195 08 558).
On the other hand, it is known that a presence of propane in the reaction gas mixture promotes the undesired formation of propionaldehyde and/or propionic acid in an undesired manner (cf., for example, WO 01/96270). WO 01/96270 therefore recommends the dilution of the reaction gas mixture with molecular nitrogen and the use of air, for example, as the oxygen source. One advantage of this procedure will simultaneously be the use of a comparatively economic oxygen source. A further objective of the processes in the prior art is to be able to use, as the propene source, a heterogeneously catalyzed oxydehydrogenation and/or dehydrogenation of propane to propylene and at the same time to dispense with a subsequent removal of the propylene formed from the unconverted propane in order thus to have available an economically viable propylene source (cf., for example, WO 03/011804, DE-A 198 37 517, DE-A 198 37 519, DE-A 198 37 520, WO 01/96370, DE-A 102 45 585 and WO 03/76370).
For this purpose, WO 01/96270 recommends a heterogeneously catalyzed propane dehydrogenation with comparatively low propane conversions.
On the other hand, for example, both EP-A 990 636 and EP-A 10 70 700 require maximum propene contents in the starting reaction gas mixture of a propene partial oxidation to acrolein in order to achieve maximum space-time yields of target product.
High propane conversions do indeed promote the possibility of establishing increased propylene contents in the starting reaction gas mixture of the propylene partial oxidation. However, they are disadvantageous in that firstly the remaining propane fraction is reduced, which has an unfavorable effect on the hotspot formation and the explosion behavior, and the secondary component formation (for example H2 or H2O) accompanying the dehydrogenation or oxydehydrogenation simultaneously increases with high propane conversions. When the resulting dehydrogenation and/or oxydehydrogenation mixture is used in this case as such for the generation of the starting reaction gas mixture of the subsequent propylene partial oxidation, as recommended, for example, by EP-A 117 146, DE-A 33 13 573 and U.S. Pat. No. 3,161,670, the result is a comparatively voluminous starting reaction gas mixture. From the point of view of maximum economic viability of the conveying of the reaction gas streams, however, small volumes are advantageous. This is especially true where high hourly space velocities on the catalyst charge according to the principle laid down in the documents WO 04/85365, WO 04/85367, WO 04/85369, WO 04/85370, WO 04/85363, WO 00/53559, WO 04/85362, WO 00/53557 and DE-A 199 48 248 are to be realized.
On the other hand, high dehydrogenation or oxydehydrogenation conversions are favorable for the purposes of very economically viable propylene generation. Conversely, an increased propene content in the starting reaction gas mixture entails an increased molar ratio of molecular oxygen to propylene therein in order to allow the activity of the catalyst charge to be developed optimally.
At the same time, too high a residual oxygen content in the product gas mixture of partial oxidation is found to be disadvantageous in that, in the case of an advantageous recycling of the residual gas which comprises unconverted propane and remains after removal of the target product from this product gas mixture into the heterogeneously catalyzed propane dehydrogenation employed for propylene generation, it undesirably attacks the propane to be dehydrogenated and lowers the selectivity of propene formation. Generally, preference is given to a heterogeneously catalyzed propane dehydrogenation over a heterogeneously catalyzed propane oxydehydrogenation.
In contrast to the exothermic heterogeneously catalyzed oxydehydrogenation which is forced by the presence of oxygen and in which free hydrogen is neither formed as an intermediate (the hydrogen pulled from the propane is pulled out directly as water (H2O)) nor is detectable, a heterogeneously catalyzed dehydrogenation refers to a (“conventional”) dehydrogenation whose thermal character, in contrast the oxydehydrogenation, is endothermic (an exothermic hydrogen combustion may be included in the heterogeneously catalyzed dehydrogenation as a subsequent step) and in which free molecular hydrogen is formed at least as an intermediate. This generally requires different reaction conditions and different catalysts from the oxydehydrogenation.
In this document, fresh propane refers to propane which has not yet taken part in any chemical reaction. In general, it will be crude propane (which preferably fulfills the specification according to DE-A 102 46 119 and DE-A 102 45 585) which also comprises small amounts of components other than propane.
In this document, the starting reaction gas mixture for the propene partial oxidation to acrolein appropriately likewise fulfills the specifications recommended in DE-A 102 46 119 and DE-A 102 45 585.
The hourly space velocity on a catalyst bed, catalyzing a reaction step, of (starting) reaction gas mixture refers in this document to the amount of (starting) reaction mixture in standard liters (=l (STP); the volume in liters that the appropriate amount of (starting) reaction gas mixture would take up under standard conditions (° C., 1 bar)) which is conducted per hour through one liter of fixed catalyst bed.
The hourly space velocity may also be based only one constituent of the (starting) reaction gas mixture. In that case, it is the amount of this constituent in l (STP)/l·h which is conducted through one liter of fixed catalyst bed per hour (pure inert material charges are not included in the fixed catalyst bed).
In this document, an inert gas refers to a reaction gas constituent which behaves substantially inertly under the conditions of the appropriate reaction and, each inert reaction gas constituent viewed alone, remains chemically unchanged to an extent of more than 95 mol %, preferably to an extent of more than 99 mol %.
A disadvantage of the recommendations, compiled above, of the prior art for a heterogeneously catalyzed partial oxidation of propene to acrolein is that they each highlight only individual aspects. It is therefore an object of the present invention to provide an improved process for the heterogeneously catalyzed partial oxidation of propene to acrolein, which takes into account the different individual aspects detailed in the prior art in their entirety in an optimizing manner.